The present invention relates to a structure of an apparatus for pulling a single crystal by the Czochralski method (CZ method), which is for growing a single crystal ingot by the CZ method.
An example of conventional CZ method single crystal pulling apparatus used for, for example, the production of a semiconductor silicon single crystal will be explained with reference to FIG. 3. As shown in FIG. 3, this CZ method single crystal pulling apparatus 30 is provided with a chamber (pulling chamber) 31, crucible 32 provided in the chamber 31, heater 34 disposed around the crucible 32, crucible holding shaft 33 for rotating the crucible 32 and rotation mechanism therefor (not shown), seed chuck 6 for holding a silicon seed crystal 5, wire 7 for pulling the seed chuck 6, and winding mechanism (not shown) for rotating and winding the wire 7 and constituted by them. AS for the crucible 32, a quartz crucible for accommodating a silicon melt (molten metal) 2 is provided inside, and a graphite crucible is provided outside the quartz crucible. Further, heater insulating material 35 surrounds the outside of the heater 34.
Now, the method for growing a single crystal by using the aforementioned CZ method single crystal pulling apparatus 30 will be explained hereinafter. First, a silicon polycrystal raw material of high purity is melted in the crucible 32 by heating it to a temperature higher than the melting point (about 1420xc2x0 C.). Then, a tip end of the seed crystal 5 is brought into contact with or immersed into the surface 3 of the silicon melt 2 at its approximate center portion by reeling out the wire 7. Then, the crucible-holding shaft 33 is rotated in an optional direction, and the seed crystal 5 is simultaneously pulled upwardly by winding up the wire 7 with rotating the wire to a direction reverse to the rotation direction of the crucible 32 to start the growing of single crystal. Thereafter, a single crystal ingot 1 approximately in a cylindrical shape can be obtained by appropriately controlling the pulling rate of the wire 7 and the melt temperature.
Although both of the quartz crucible and the graphite crucible provided in the aforementioned single crystal pulling apparatus have high heat resistance, they have a drawback that they are rather brittle and thus shows poor shock resistance. For this reason, when polycrystal raw material is charged in a crucible and melted by heating in pulling of a single crystal, a lump of the polycrystal may collapse during the melting and the crucible may be broken by its impact to generate cracks, through which the melt may leak. Further, while the multiple CZ method is widely used to attain efficient single crystal growth, in which multiple single crystal ingots are obtained from a single quarts crucible by not solidifying the melt after pulling of a single crystal and charging the polycrystal raw material again, it is expected that the silicon melt may flow out of the crucible during the operation because of breakage of the crucible, scattering of the melt and so forth upon the recharging of the polycrystal raw material. Further, although it would be a rare case, if a big quake is brought to the pulling apparatus by earthquake or the like, it is expected that the pulling apparatus is largely swung to right and left and thus the melt in the crucible may flow outside of the crucible. Furthermore, when the crucible has been gradually degraded by use or a single crystal falls during the pulling, the crucible may be broken and the substantially whole amount of the melt contained in the crucible may flow out.
If the melt at a high temperature flows out or scatters to the outside of the crucible as described above, it reaches the bottom of the chamber from the circumference of the crucible and erode the bottom portion of the chamber, metal parts such as terminal area for heater and the crucible holding shaft, crucible driving apparatus, lower piping for cooling water provided for cooling the chamber and so forth. In particular, since the silicon melt heated to a high temperature shows strong erosion action against metals, it is also expected that the piping for cooling water may be eroded. Moreover, the melt overflowed outside of the chamber will harmfully affect operators and facilities.
Therefore, in the single crystal pulling apparatus disclosed in Japanese Patent Laid-open (Kokai) Publication No. 9-221385, for example, it is attempted to obviate such problems by providing a catch pan for melt leakage having an inner volume that can accommodate the whole melted raw material under the crucible, which pan is formed as a seamless pan by integral molding.
However, in such a catch pan for melt leakage, the bottom and side of the pan are completely integrally formed. Therefore, as the single crystal to be grown becomes larger and thus the single crystal pulling apparatus becomes larger, such a pan is becoming extremely expensive in view of its material. Further, in the case that it is integrally formed by press-fitting the bottom portion and side portion of the pan to each other (contacting surfaces are tapered and fitted to each other), the side portion may be lifted up from the fitted position when the press-fitting is not so strong and the silicon melt is accumulated in the catch pan, because the specific gravity of silicon (melt) is 2.54 and specific gravity of graphite is around 1.9. Thus, the melt is highly likely to flow out.
Thus, the present invention is accomplished in view of such problems of the conventional techniques, and its major object is to provide a catch pan for melt leakage provided in a CZ method single crystal pulling apparatus, which can, even if a melt flows out of the crucible by a certain possible cause, prevent the melt flowed out from reaching the lower mechanisms including metal parts, piping and so forth, and thereby prevent bad influences on operators and peripheral equipments.
In order to achieve the aforementioned object, the catch pan for melt leakage in a single crystal pulling apparatus according to the present invention is a catch pan for melt leakage provided under a crucible at a bottom portion of a chamber in a single crystal pulling apparatus based on the Czochralski method, wherein the catch pan for melt leakage comprises at least a bottom portion and a barrel portion, and the bottom portion and a barrel portion are connected by screw-fitting or by using a tap bolt.
If a catch pan for melt leakage is constituted as described above, even a large catch pan can be produced with parts made from smaller raw materials compared with integrally formed product, because it is assembled from at least two parts, the bottom portion and the barrel portion, by screw-fitting or by using a tap bolt. Therefore, material yield of graphite material is markedly improved, and thus the production cost can be reduced. Further, even if the melt flows out of the crucible and accumulates in the catch pan, the catch pan is not separated into each of its parts, for example, the barrel portion does not float, and thus leakage of the melt out of the catch pan can surely be obviated.
In this case, the catch pan for melt leakage can comprise a catch pan body made of graphite material and a heat insulating material adhered to an internal surface of the body.
If the catch pan is constituted as described above, strength and configurational precision of the catch pan is secured by the catch pan body made of the graphite material, and heat transfer from the leaked and accumulated melt at a high temperature is suppressed by the heat insulating material adhered to the internal surface of the body. Thus, thermal load on the chamber can be reduced, and breakage of the chamber can be obviated.
Further, in this case, the graphite material can comprise isotropic graphite, and the heat insulating material can comprise a molding material made of carbon fibers.
If isotropic graphite is used for the graphite material as described above, the catch pan would have excellent strength and heat resistance as well as high configurational precision and excellent configurational stability. Therefore, it can be a catch pan for melt leakage that can be made also by screw-fitting. Further, if a molding material made of carbon fibers is used as the heat insulating material, the heat insulating material can be integrally formed in a shape fitted to the shape of the catch pan body made of graphite material, or it can be easily formed by separately molding each of its parts and jointing them.
Furthermore, in this case, the catch pan for melt leakage preferably has an internal volume that can accommodate at least the whole melted raw material.
In such a case, even if the substantially whole amount of the melt flows into the catch pan due to breakage of the crucible, falling of a single crystal or the like, the melt does not leak out of the catch pan, and thus the lower mechanisms in the chamber or operators and equipments outside the chamber can be protected. In this case, as also understood from the fact that the specific gravity of silicon melt is larger than that of solid silicon, volume expansion of solidified silicon must be sufficiently taken into consideration.
Further, in a single crystal pulling apparatus provided with such a catch pan for melt leakage as described above, even if the melt flows out of the crucible, it can be prevented from reaching the lower mechanisms including metal parts, piping and so forth, and a single crystal can be pulled by a single crystal pulling method utilizing the single crystal pulling apparatus provided with the catch pan for melt leakage without giving bad influences on operators or peripheral equipments.
As explained above in detail, according to the CZ method single crystal pulling apparatus of the present invention, even if a silicon melt flows out of the crucible by a certain possible cause, it can be completely received by the catch pan for melt leakage, which is preferably made of isotropic graphite and adhered with a heat insulating material, and it can be prevented from leaking to the outside. Therefore, there can be obtained an advantage that breakage of chamber can preliminarily be obviated by preventing the melt from reaching metal parts such as piping for cooling water.
Further, since the catch pan for melt leakage is assembled by connecting at least a bottom portion and a barrel portion, raw materials can be easily supplied at a low cost, and it can meet to production of a larger single crystal pulling apparatus. Moreover, since the catch pan has a simple structure and it can be easily produced, the cost can be reduced. Furthermore, since a heat insulating material is attached inside the catch pan, heat quantity effused from the catch pan is small even if melt leakage occurs. Therefore, thermal load on the chamber can be reduced and thus breakage of the chamber and so forth can be obviated.